This invention relates broadly to structural members, such as studs used for structural framing of building systems, i.e., beam columns, etc., and particularly to casement sections, or frames and sashes for windows, doors, panels or other building or architectural constructions. For convenience herein, the invention will be defined and described with particular reference to window casements (frames and sashes), but it will be understood that the invention is of general utility.
Existing window casement members used in the construction of window frames and sashes suffer from a number of disadvantages and it is difficult to meet all the practical requirements without excessive weight or cost. For example a window casement must have adequate strength, the corners and other joints must be rigid, the casement members must have longitudinal and torsional rigidity, the members must be durable, resistant to impact and damage, corrosion resistant, leakproof, remain stable at temperatures above 150.degree. F., and also preferably should have thermal insulating properties to prevent excessive conduction of heat from the internal to the external surfaces, or vice-versa.
Traditional timber and extruded aluminum and polyvinyl chloride (PVC) and other composite material members fail to meet a number of these requirements, while conventional solid or hollow extruded aluminum or pultruded glass resin shapes are expensive and also fail to meet many of the requirements. Window casement members have also been constructed in hollow rolled aluminum brass or steel sections, which may be coated to resist corrosion, but in order to provide adequate longitudinal and torsional strength the gauge of the rolled sheet must be quite substantial, which results in a heavy and expensive construction. Moreover, it is difficult to form corner and other joints since steel members coated with material such as synthetic plastics or pultrusions cannot be welded without damaging the coating.
Various other composite constructions have been proposed but all known constructions are either expensive or fail to meet the optimum design requirements.
Prior art conventional PVC frames are a faulty combination of low rigidity with poor material shear strength that requires an aluminum or steel stiffener be placed into one of the internal cavities to enhance its structural properties. The rigidity enhancement necessary is minimized by locating the structural member close to the neutral axis of the PVC shape.
Additional mechanical problems with the conventional PVC design include excessive thermal expansion with temperature fluctuations from winter to summer and extreme temperature gradients from the environmentally controlled inside of the building to the natural environment of the outside of the building, where a typical window will expand and contract 1/4 inch in height and width. This places stress on the welded corner joints and nailing fin which has proven to cause fracturing failures in the field. Additionally, the PVC shape is generally restricted to lighter colors such as white and beige, as darker colors can allow the PVC temperatures to rise above the heat deflection temperature of the material, which is approximately 140.degree.-150.degree. F.